There can be many causes of head gasket failure, the usual being block or head faces that are not completely flat (this includes localised low spots such as between cylinders on blocks), cylinder head lift due to inadequate head bolt clamping, abnormal running conditions such as detonation or extermely high temperatures, and of course the use of substandard headgaskets and components.

I cannot comment on any of the above problems on your engine, except to say you are using a very low quality head gasket, so that alone could be your problem. Use a genuine Ford/Payen head gasket and see how it goes. If it blows with that fitted you have one or more of the issues Ive listed above!!